1. Field of the Invention
Embodiments of the invention relate to variable resistance semiconductor memory devices. More particularly, embodiments of the invention relate to a resistance random access memory (RRAM) having a common source line and an associated data access method.
2. Discussion of Related Art
Next generation nonvolatile memory devices are being developed for use in portable consumer products to provide high capacity and low power consumption. Next-generation memory devices include, for example, PRAMs (Phase change Random Access Memory) utilizing phase-change material that can be switched between two states, RRAMs(Resistance Random Access Memory) employing material having a variable resistance characteristic of complex metal oxides, and MRAMs (Magnetic Random Access Memory) having ferromagnetic material. A common characteristic of the various materials used to form next generation memory devices is that resistance values vary by the supplied current or voltage. In addition, the resistance value is maintained in these devices even when no current or voltage is supplied demonstrating the nonvolatile memory characteristic which obviates the need for refresh operations. In these devices, a memory cell is comprised of one switching device and one variable resistor. The variable resistor is connected between a bit line and the switching device. The switching device, which is constructed of an access transistor or diode, is typically coupled with the variable resistance and a word line.
A PRAM memory device may replace NOR flash memory typically utilized in mobile phones. In PRAMs, a variable resistor is formed of a chalcogenide alloy of germanium, antimony and tellurium (Ge—Sb—Te) called GST and the resistance of the material is changed by a change in temperature. When the variable resistor is formed of complex metal oxides disposed between top and bottom electrodes the device is most likely a RRAM. When a variable resistor is formed of an insulator disposed between top and bottom magnetic electrodes, the device may be an MRAM.
A data write operation in a PRAM is performed by applying voltage pulses having the same polarity with different amplitudes to a memory cell at different times. A phase change memory device produces a switch based on a setting and resetting temperature which causes the device to be sensitive to a change in neighboring temperature. On the other hand, in an RRAM device, material which has a characteristic resistance value is changed by the applied voltage or current to store information in a memory layer. Thus, two electrodes are provided with the memory layer and voltage or current is applied to the two electrodes. This simple structure reduces the size of the memory device.
FIG. 1 is a schematic diagram illustrating a conventional memory cell MC of an RRAM. Memory cell MC constitutes one cell having an access transistor AT and a resistive memory device SE. The resistive memory device SE may be formed of a complex metal oxide VR disposed between a top electrode TE and a bottom electrode BE. The complex metal oxide may be chrome doped SrZrO3 or a thin film of polycrystal PrCaMnO3 material. The gate of access transistor AT is connected to word line WL, its source is connected to source line SL, and its drain is connected to bottom electrode BE of resistive memory device SE. Bit line BL is connected to top electrode TE. When data of a first state is written to memory cell MC current is applied in a first direction D1. When data of a second state is written to the memory cell MC is applied in a second direction D2. The RRAM is also called a bi-directional RRAM in that writing data of a first and second state is performed in mutually opposite directions. The first direction D1 indicates a current path where a positive voltage applied from the bit line reaches source line SL via memory cell MC, and may be referred to as a first write path. The second direction D2 indicates a reverse current path (second write path) where a voltage applied from source line SL reaches bit line BL via memory cell MC. When writing data through the first write path, the resistive memory device SE has a high resistance value. When writing data through the second write path, the resistive memory device SE has a resistance value which is lower by hundreds or thousands of times as compared with the resistance value associated with the first write path.
Data can be read from memory cell MC via the use of a sense amplifier, coupled to bit line BL that senses current flowing through the bit line. The through-current varies depending on whether the memory cell is under a set state ‘1’ or under a reset state ‘0’. For example, the amount of through-current may be small because of a high resistance when the memory cell has a ‘set’ state and may be relatively large because of a low resistance when the memory cell is in a ‘reset’ state. Accordingly, the sense amplifier compares the through-current to the reference current to read data stored in the memory cell MC.
FIG. 2 illustrates a sectional view of a plurality of memory cells of FIG. 1 fabricated on a P substrate and its corresponding schematic circuit. A PCMO film 1 corresponds to VR and the platinum (Pt) layer forms the top electrode TE and bottom electrode BE. NMOS transistor 2, which is formed on a P-type substrate and whose gate is coupled to a word line, corresponds to access transistor AT Fabrication details of the memory cells shown in FIG. 2 are disclosed in U.S. Pat. No. 7,057,922. The plurality of memory cells shown constitute a memory cell array having a matrix of rows and columns (as shown in FIGS. 3 and 4) to have a high density memory capacity.
FIGS. 3 and 4 illustrate conventional memory cell array configurations. The cell array of FIG. 3 (the details of which are disclosed in U.S. Pat. No. 7,045,840) includes a source line disposed in the direction of a word line. However, different source lines are correspondingly disposed for memory cells coupled with different word lines. The cell array of FIG. 4 (the details of which are disclosed in U.S. Pat. No. 7,016,222) includes a source line disposed in the direction of the bit line which is perpendicular to a word line. The memory cell arrays as illustrated in FIGS. 3 and 4 are RRAM type devices where the layout of word lines and bit lines occupy valuable chip area. By reducing the number of source lines in the RRAM device, the utilized chip area would correspondingly be reduced.
In a write operation of FIGS. 3 and 4, it is difficult to keep a source line potential and a bit line potential within a positive voltage level range. For example, when writing data 1, a ground voltage is applied to the source line and a positive write voltage is applied to the bit line. When writing data 0, a ground voltage is applied to the source line and a negative write voltage is applied to the bit line. When writing data 0, a negative voltage generated from a negative voltage generator must be applied to the selected bit line. This requires installing a negative voltage generator within the memory chip. Accordingly, there is a need for an advanced layout structure of a RRAM memory cell array and associated data access method.